As automobiles have come to have higher performances and have become more diversified, property requirements for tire have changed accordingly and a need has become high for an all-weather tire balanced in properties such as high grip, low fuel consumption, low-temperature properties in winter, and the like. Hence, it is desired to develop a rubber material balanced in such properties.
There were proposed, as such a rubber material, for example, blends of natural rubber (NR) or a polyisoprene rubber (IR) and a styrene-butadiene copolymer rubber (SBR) or a high-cis polybutadiene rubber (CBR). They, however, have inferior wet skid property and, when using a SBR of high styrene content, have low grip at low temperatures and cause cracking of tire tread portion, etc.; thus, they have no satisfactory requirements. There were also proposed blends of a polymer of high glass transition temperature (high Tg) (for example, a SBR of high styrene content) and a polymer of low Tg [for example, a high-cis polybutadiene (CBR)]. They, however, are low in tensile strength property and inferior in balance between wet skid and low-temperature properties. There were also proposed, as a polymer having properties of both a high-Tg polymer and a low-Tg polymer, block polymers composed of said two polymers as block components [e.g. Japanese Patent Publication No. 37415/1974, Japanese Patent Application Kokai (Laid-Open) No. 102912/1982, Japanese Patent Publication No. 21702/1992, Japanese Patent Publication No. 61900/1992, EP 54,440 and U.S. Pat. No. 4,463,133]. The block copolymers certainly have improved low-temperature properties, but are inferior in tensile strength and abrasion property.